The present invention relates to an X-ray diagnostic system which produces an X-ray picture for diagnosis of a patient and a method of picking up an X-ray picture employed in this X-ray diagnostic system, both of which produce a quality X-ray picture.
An X-ray diagnostic system both of generally comprises an X-ray generator, an image intensifier (abbreviated as "I.I." hereinafter), an imaging device such as a pickup tube or a solid-state image sensor (including a CCD [Charge Coupled Device]), and a television monitor.
The X-ray generator is furnished with an X-ray tube generating X-rays whose intensity depends upon the X-ray tube voltage and the X-ray tube current. X-rays generated by the X-ray generator pass through a patient and enter the I.I. The I.I. converts the incident X-rays to fluorescent light in proportion to the X-ray transmittance in each part of the patient. The fluorescent light exits the I.I. and goes into the imaging device.
An optical system comprising an iris and tandem lens unit including a plurality of lenses can be disposed between the I.I. and the imaging device so as to have the fluorescent light form an image of the patient at the imaging device. The iris is usually arranged before a rearmost lens of the tandem lenses which causes the fluorescent light to go through its adjustable aperture. The tandem lenses converge the fluorescent light toward the imaging device.
The imaging device is controlled by a CCU (Camera Control Unit) and converts the incident fluorescent light to an electric signal (image signal). Commonly, the image signal is transmitted from the imaging device to an image processor where the image signal is subject to image processing such as noise correction, contrast processing, etc.
After image processing, the image signal is sent out to the television monitor. X-ray images of the patient can be monitored continuously on a screen of the television monitor in accordance with a series of frames. A "frame" means one exposure of continuous X-ray radiography for a television monitor. A frame rate is usually 30 frame/sec. That is, one frame takes 33.3 msec.
The imaging device has its own optimum range of quantity of incident light for forming quality images. The imaging device is unable to form clear images for the quantity of light over or under the optimum range.
For this reason, part of the fluorescent light is taken out of the optical system for one frame so as to detect its quantity. The quantity of the fluorescent light taken out is detected by a quantity-of-light detector. If the quantity of light detected departs from the optimum range of quantity of light, the quantity-of-light detector informs an iris controller of this fact. The iris controller controls the degree of aperture of the iris so as to make the appropriate quantity of the fluorescent light go through a few frames later.
If the quantity of light detected departs so great from the optimum rang of quantity of light that the optimum quantity of light cannot be achieved by adjusting the aperture of the iris only, the quantity-of-light detector informs an X-ray controller of this fact. The X-ray controller controls the X-ray tube voltage or an X-ray tube current at the X-ray generator so that it generates X rays of the optimum intensity which lead to the optimum quantity of the fluorescent light. The control of X-ray intensity by adjusting the X-ray tube voltage or the X-ray tube current requires more time than the adjustment of the aperture of the iris.
Furthermore, if an intensity of X-rays having passed through patient or I.I. magnification (ratio of input size to output size) fluctuates due to a change in a patient diagnosis or for any reason, then quantity of fluorescent light generated at the I.I. changes. Thus, the quantity of fluorescent light has to be detected for each patient, for example, in order to make sure it falls into the optimum range of quantity of light.
However, since the distance between the imaging device and a rearmost lens of the tandem lenses in charge of the last image formation is short and it is difficult to take the fluorescent light from outside of the optical system, the conventional detection of the quantity of light of used to be based on the fluorescent light taken out between a plurality of the tandem lenses.
Namely, whether the quantity of fluorescent light at the imaging device is within its optimum range is judged based on whether the quantity of fluorescent light inside the optical system is within a criterion range estimated as quantity of corresponding to the optimum range of quantity of light at the imaging device which takes in absorption, reflection, etc. of the fluorescent light inside the optical system. Consequently, the judgment of the optimum quantity of light is likely to be inaccurate because of an error of the estimate of the criterion range and an X-ray image of low quality happens to be picked up.
Besides, as mentioned hereinbefore, it is impossible to achieve the estimated criterion in the first frame by the conventional way of adjusting the aperture of the iris and it demands a few frames to realize a desired aperture. Accordingly, a test X-ray exposure which requires some frames has to be performed to determine the estimated criterion prior to pick up an X-ray image for actual diagnosis of a patient, and the patient is exposed to more X-ray.